Sophisticated, critical electronic systems require power supplies evidencing the attributes of very high quality and reliability. These attributes generally are not available from the power supplied through typical utility organizations. As a consequence, the users of such systems have resorted to various techniques for power input improvement. The more elaborate approach to power supply improvement is provided by uninterruptible power supply (UPS) systems. These systems exhibit a desirably high level of power assurance, providing stand-by or "ride through" power in the event of complete line outage as well as providing waveform regulation. Typically, the design approach taken for UPS systems includes the provision of rechargeable batteries as a first level of stand-by power. These batteries are associated with an a.c. utility input through a d.c. node level generated by rectifying and treating the line input. The thus-developed d.c. energy then is converted to an a.c. output by an appropriate inverter and filter stage or stages. Such inverter typically are elaborate devices due particularly to the waveform regulation features incorporated in them. Generally in a UPS arrangement, the inverter component of the system is continually "on-line" or operating in serial association with the power path. Because these inverter devices may fail from time-to-time, their performance usually is monitored and a rapid switching technique is provided which employs solid-state switches to respond to such failure and transfer the system to direct utility power. The "static switches" used for this switching function conventionally employ silicon controlled rectifier (SCR) devices operating in conjunction with a control circuit functioning to gate them into conduction and to force commutate them off, when necessary, i.e. the solid-state devices are turned off by, in effect, reversing current through them where the time required for "natural" commutation by polarity reversal is considered excessive.
SCRs may be categorized as a form of power device, exhibiting internal regenerative characteristics such that they tend to accommodate heavy surge overloads. Correspondingly, currently produced power transistor devices, which may also be used for the instant purpose, generally are unable to accommodate power vagaries. Thus, the SCR usually is elected as the control element for the static switch function of UPS systems, and commutation considerations usually accompany design requirements. Utility derived power treatment not incorporating stand-by power supplies as above discussed conventionally has been provided by motor generators or voltage synthesizers. The former devices are electric motor driven generators wherein utility power is applied to the prime mover or motor and the resultant power output of the driven generator is one of controlled waveshape. Because of the mechanical inertia involved in such a combination of components, aberrations in input waveshape and the like are not transmitted through the system. However, these devices necessarily incorporate moving mechanical components which eventually will wear and fail without adequate surveillance.
Recently, a polyphase ferroresonant voltage stabilizer or synthesizer has been successfully introduced to the marketplace. In their elementary form, such synthesizers comprise a regulator which is fashioned as a non-linear saturable transformer arranged in parallel with a capacitor assemblage which is supplied from the line source through an input inductor. The saturable transformer components and capacitors form a ferroresonant circuit wherein the reactive components operate beyond the knee of a conventional magnetization curve. Described in U.S. Pat. No. 4,305,033 by Jeffrey M. Powell, the noted ferroresonant voltage stabilizer or synthesizer enjoys advantages of economic construction and efficient performance while remaining immune from certain unsatisfactory characteristics related to stability and reliability which previously had been associated with resonating circuits. Improvement in terms of operational efficiency, size and cost for such synthesizers are described in U.S. Pat. No. 4,544,877 by Powell. While voltage synthesizers such as the Powell apparatus have experienced important acceptance in the marketplace, particularly in conjunction with computer room installations, the devices as presently constituted do not have the capability of providing stand-by power in the event of utility line power outage. As the user of these devices have expanded their facilities requiring assured power inputs, a need has arisen for enhancing the synthesizer so as to develop a UPS capability within reasonable cost constraints.